1. Field of the Invention
The present invention relates to a process for prospecting the ocean bed by measuring electromagnetic fields and to a device for implementing the process.
2. Description of the Prior Art
Attempts have been made to use methods such as those employed on land which are based on the existence of contrasts in electrical resistivity between the different materials forming the subsoil for prospecting for mineral resources situated under water, more particularly, below the ocean and, more generally, for all geological studies of ground covered by water. However, although these methods are commonly employed on land, many difficulties arise when they are employed on the water and hitherto these have prevented their widespread use.
If electrical dragging or sounding techniques using continuous current are employed, the high conductivity of sea water tends to shortcircuit the current injection electrodes which makes it impossible to carry out thorough investigations of the underlying soil formations.
The use of electromagnetic prospecting processes employing aircraft, which are generally employed on land, would be simple to implement and would enable vast areas to be covered rapidly but, once again, when these processes are used over the ocean it is necessary to overcome the problem of the conductive layer of water which forms an impassable screen for the rays at the frequencies employed which are relatively high.
To overcome this problem it was sought to use frequential electromagnetic sounding processes at very low frequency such as the magneto telluric process which employs natural emissions. However, the equipment required for this process is so complicated and the recording periods at a particular station are so long that the process is rendered so expensive and delicate that its practical application is extremely problematical.
With regard to electromagnetic sounding process employing a dipole emitting low frequency rays and a suitable receiver, simply the conversion of these devices to adapt them to marine prospecting conditions produces a number of difficulties: the difficulty of providing fixed stations, the need to use low frequencies which are not particularly compatible with the movement of a stationary ship or a ship moving at a given rate, the need to use two boats each time the distance between the emitter and the receiver is too great to be contained on a single boat and each boat requires its own equipment and at least one operator. In addition, these two boats would have to maintain a constant distance between one another and follow the same route. These operations are extremely difficult to carry out in practice.